Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010

The Cachiyacu Geothermal Prospect, Chacana Caldera, Ecuador

Bernardo Beate(1), Salvatore Inguaggiato(2), Fabian Villares(1), Stalin Benitez(3) and Silvana Hidalgo(4). (1) Dto. de Geologia, Escuela Politecnica Nacional, P.O. Box 17 01 2759, Quito / Ecuador (2) Istituto Nazionale di Geofisica e Vulcanologia -Palermo, via Ugo La Malfa 1590146, Palermo, Italy (3) Samanes 1ª. Etapa, Manzana 130, Villa 1, Guayaquil, Ecuador (4) Instituto Geofisico, Escuela Politecnica Nacional, Ladron de Guevara E11-253 y Andalucia, Quito, Ecuador [email protected] [email protected] [email protected] [email protected] [email protected]

Keywords: Ecuador, geothermal exploration, resurgent the Eastern Cordillera (CR), about 65 km ESE from the caldera, rhyolite dome, hot springs, use of geothermal capital city of Quito. Elevation range from 3200 up to 4100 energy. masl on ragged topography covered with high-altitude grassland and few cloud-forest patches. Climate is cold and ABSTRACT wet for most of the year. There are no people living in Cachiyacu area and there is no road access to it, although Cachiyacu is located at about 65 km ESE of the capital city the villages of El Tambo and Papallacta are 5km and 12 km of Quito, in the high ranges of the Eastern Cordillera at away in NE direction, respectively. A paved highway 4000 masl, on ragged topography in a wet and cold climate. connects these villages with Quito and the Amazon basin. Paved accesss road and high voltage transmission line cross along its northern border; the main load center is Quito. The Earlier work on the assessment of geothermal resources by heat source is the southern part of the resurgent silicic INECEL-OLADE (1980) and by Almeida (1990) and Chacana caldera, which has been persistently active since Almeida et al., 1992, didn’t consider Chacana as a high late Pliocene, producing numerous rhyolitic domes, temperature geothermal resource, although it is taken into andesitic to dacitic lava flows and ignimbrites of high-K account by Beate & Salgado, 2005, and Beate, 2008, which calc-alkaline affinity; latest volcanic activity comprises two prompted the interest of Electroguayas in the area. The large Si-andesite lava flows as recent as 240 yr BP. The 35 actual knowledge is mostly regional geology; recent by 15 km diameter caldera structure is hosted in the vulcanological studies by IG/EPN by Hall & Mothes (2008) Paleozoic-Mesozoic metamorphic basement and is likely have outlined the evolution of Chacana Caldera and its roll controlled by regional NNE-striking strike-slip faults. A hot as part of the Ecuadorean Rhyolitic Province (ERP). Just water–dominated geothermal system, with deep recently, the geochemical base line for the Ecuadorian arc temperatures in excess of 200 °C is assumed to be hosted in has been established, including the Chacana area the late Tertiary to Quaternary volcanic pile filling the (IG/EPN/INGV 2009) caldera and in its lower wall rocks. 30 to 65 °C, highly saline hot springs are common inside the area and its waters In the following sections we present an outline of the belong to the Cl-SO4-alkaline type. Hot spring water along regional geological setting of the Chacana Caldera, a brief a NE-oriented outflow at 5 to 10 km distance, is used sketch of the geology and evolution of the Chacana Caldera mainly for bathing purposes and less for space heating. with emphasis on its southern part and Cachiyacu in Older hydrothermal alteration is widespread in the area. A particular; than we outline the main results of the preliminary geothermal model is proposed in the Cachiyacu geochemical survey on hot spring waters and discuss the area with a self-sealed reservoir and permeability sustained mayor issues of a preliminary geothermal model for by active faults. A comprehensive magnetotelluric survey is Cachiyacu. recommended to locate the geothermal reservoir and to site the first deep drilling targets in Cachiyacu. 2. GEOLOGICAL SETTING Continental mainland Ecuador consists of three distinct 1. INTRODUCTION geomorphologic and geographic regions: the coastal plains The Ecuadorean government has priorized the development or COSTA, the Andes mountain chain or SIERRA and the of renewable sources of energy with the aim to reduce and Amazon basin or ORIENTE; a fourth region comprises the eventually replace the use of fossil fuels for power Galapagos Islands located about 800 km to the W of the generation. This study was funded by Electroguayas, a state coastline in the Pacific Ocean (see fig. 1). owned power generation entity, with the objective to assess the geothermal potential of the Cachiyacu area and prepare The Andes constitute the very backbone of the country. Its a preliminary geothermal model, based on reconnaissance formation is due to multiple accretion since Jurassic times geological and geochemical surveys. Geological work (Aspden and Litherland, 1992; Eguez and Aspden, 1993). It included field mapping and sampling, as well as consists of two parallel NNE-striking mountain chains: a) geochemical assays by ACME Labs, Vancouver-Canada of the Eastern Cordillera or Cordillera Real (CR) which are the most representative rock samples. Geochemical work sublinear belts of Pz-Mz metamorphic rocks intruded by consisted of sampling water from 19 warm and hot springs both, I- and S-type granitoids of early Mesozoic age; and b) as well as bubbling gas and surface waters; geochemical the Western Cordillera or Cordillera Occidental (CO), analysis of the fluids were done at the Istituto Nazionale de which consists of late Mesozoic to early Cenozoic basalts Geofisica e Vulcanologia at Palermo, Italy. and vulcaniclastics, representing accreted oceanic terrains (Hughes and Pilatassic, 2002). These rocks are intruded by The Cachiyacu geothermal prospect is located in the Tertiary granitoids. Both cordilleras have been folded and southern end of the silicic Chacana caldera, on the crest of uplifted and are covered by late Tertiary volcanics. 1 Beate et al.

Fig. 1 Geodynamic setting of Ecuador, depicting location of Cachiyacu Geothermal Prospect

The Interandean Valley (IAV, see fig. 1) is located between Geodynamic processes in Ecuador since late Oligocene are the two cordilleras and comprises thick late Tertiary to controlled by the near orthogonal convergence between Recent vulcaniclastic and epiclastic sedimentary sequences; Nazca and Southamerican plates, which has generated it is bounded laterally by NS oriented terrain sutures, the regional uplift and crustal faulting and deformation as well Pujili – Calacali suture to W and the Peltetec suture to E, as extensive volcanism (Londsdale, 1978). which continue into Colombia as the Cauca-Patia and Romeral sutures, respectively. Suture-paralell overprinting The compressive tectonic regime formed several active faults, mostly thrust faults, mark the morphology of intramontane basins between the two cordilleras since the actual IAV borders. Miocene. The IAV has been formed as a spindle shaped basin along a restraining bend in a transpressive regime Covering both cordilleras in its northern half, a well since about 6 Ma due to an increase in the coupling of the developed, subduction-related, broad, calc-alkaline Carnegie ridge in the subduction zone (Spikings, 2001; continental volcanic arc extends northwards into Colombia Winkler, 2002; Villagomez, 2002). (Barberi et al., 1988; Hall & Beate, 1991). The arc is of Quaternary age and consists of more than 50 volcanoes, of The northern half of the country is now part of the North which at least 30 are active and four are in eruption or did Andean Block (NAB), which moves at 6-10 mm/yr in NE- so in the last ten years. The southern half of the Ecuadorian NNE direction along regional dextral strike-slip faults Andes shows only extinct volcanic activity, the latest one (Ego, 1993). This active regional fault system enters the being at Quimsacocha caldera at 3.6 Ma (Beate et al., gulf of Guayaquil creating extension and normal faulting 2001); the outset of volcanism is due to the flattening of the (fig.1). It continues northwards along the dextral Pallatanga slab since late Miocene (Gutscher et al., 2000). fault, which cuts both, the Western (CO) and the Eastern Cordillera (CR), as well as the IAV, emerging further N as The Oriente is an extensive sedimentary basin, overlying the Chingual fault at the border with Colombia, where it cratonic basement (Baldock, 1982). Older rocks comprise continues along a NS strike. Jurassic volcanics and batholiths and a Cretaceous carbonatic platform, covered by Tertiary epiclastic This tectonic activity together with the extensive sediments; large NS-striking thrust faults cut the sequence Quaternary volcanism present in the northern Ecuadorean toward its western limit (Tschopp, 1953). Quaternary Andes are the regional setting for the Chacana caldera and alkalic volcanoes are located along the W margin of the the Cachiyacu geothermal prospect. basin in a back-arc setting. 3. GEOLOGY OF CACHIYACU PROSPECT The Costa is the flat region between the Andes and the The Cachiyacu prospect area is located inside the Chacana Pacific Ocean. It comprises a late-Cretaceous to Cenozoic caldera, close to its southern rim and comprises about 100 fore-arc basin underlain by mid to late Mesozoic oceanic km2. The area is flat caldera floor at the site of the hottest crust (Benitez, 1995; Vallejo, 2009). There is no active thermal springs, where it is surrounded by steep caldera volcanism in this area. walls to W, Plaza de Armas dome-flow complex to the S The Galapagos Islands are an active mantle hot spot and the and Yanaurcu and San Clemente felsic domes to E and N, asismic Carnegie ridge (see fig. 1) represents the trace of respectively. In contrast, topography to the E of the the hot spot over the Nazca Plate, which is actively mentioned domes is rugged and difficult. subducting underneath continental Ecuador, producing most of its volcanism and intense seismic activity.

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Fig. 2 Geologic map of Cachiyacu

The Chacana caldera is the largest rhyolitic eruptive center km S of Chacana, INECEL 1983, Beate-1985 and in the northern Andes (Hall & Beate, 1993; Hall and Hammersley, 2003) and many rhyolitic domes and felsic Mothes, 2001; 2008) and constitutes the northern part of the intrusions; it is located on a 50 to 70 km thick continental Ecuadorian Rhyolite Province (ERP), proposed by Hall & crust, in concordance to a – 210 mgal Simple Bouguer Mothes (2008). This province covers an area of about 100 x gravity anomaly low (Feininger and Seguin, 1983). 40 km, along the crest of the Cordillera Real (CR), and consists of large silicic collapse calderas like Chacana (diameter 35 x15 km) and Chalupas (diameter 15 km, 40 3 Beate et al.

The Chacana pre-caldera edifice measures 60 km NS and At about 0.45 Ma, magmatic activity resumed vigorously 28 km EW and was built initially as an andesitic with the intrusion of a voluminous rhyolite – dacite sill or stratovolcano cluster during late Pliocene. This cluster was laccolith, which caused: a) the uplift and resurgence of the covered by voluminous radial volcanic deposits (U1 in central part of the caldera for about 500 m, b) the formation Table 1) of dominantly dacitic and rhyolitic ignimbrites, of an extensive 10 to 15 km diameter phreato-magmatic vitrophyres and lava flows until about 1 Ma. The whole diatreme breccia (p-m dbx), similar in size to the Chacana sequence is about 1 km thick; it overlies a 1-2 km Guinaoang 8.5 x 3.5 km diatreme in Philipines (Sillitoe, thick Mio-Pliocene pile of andesite flows, breccias and tuffs 1985), which partially changed the caldera floor from plate- of Pisayambo Fm. The basement to the whole volcanic to funnel-shaped, and c) the emplacement of late rhyolitic sequence consists of Paleozoic-Mesozoic metamorphic domes into the latter (unit U3 in table 1). Volcanic activity rocks to the center and E side, and Mesozoic oceanic continued until about 0.165 Ma, with the intracaldera basalts to the W side. The two basements were put together extrusion of andesitic, dacitic and rhyolitic lava flows and by accretion at 75 Ma along the NS-striking Peltetec suture domes, obsidian flows, pyroclastic flows and extensive (Spikings et al., 2005); this suture, now inactive, is the rhyolitic tephra fall deposits of regional extent (Hall & likely western control to ERP and to Chacana and Chalupas Mothes, 2008). The vents were located mainly along the calderas. structural collapse rim and on the caldera floor. Hydrothermal activity caused extensive alteration in the A second set of active regional dextral strike-slip faults caldera filling rocks. Most of the rocks cropping out at the (Soulas, 1991) runs through the Chacana area along a NNE Cachiyacu area belong to this magmatic pulse, like the direction and is likely responsible for the caldera formation Chuzalongo andesites, Plaza de Armas rhyolites and and geometry and location of subsequent volcanism. The obsidian flows and the San Clemente and Yanaurco domes Chacana caldera formed by structural collapse about 1 Ma (see map and cross section, fig 2 and 3, and U3 in Table 1). ago due to the explosive and voluminous extrusion of rhyolitic ignimbrite or ash flow, now partly preserved as In late Pleistocene and synchronous with the pleniglacial buried, distal, reworked tuffs in the sedimentary fill of the period, between 40 and 20 ka (Clapperton and Vera, 1986), IAV (the Golden Tuffs of Villagomez, 2002). The several lava flows of Si-andesites poured out inside and topographic caldera diameter is about 35 x 15 km, outside of the topographic rim of the Chacana caldera (see elongated in a N to NNE direction, and probably U4 in Table 1; Hall & Mothes, 2008). The most recent corresponds to a plate collapse geometry (Lipman, 1984). eruptions in the Cachiyacu area occurred in the last The caldera depression was partly filled during and after the thousand years (see fig. 1 and U5 in Table 1) and consist of collapse by ignimbrite, collar-landslide breccias (as Si-andesites; the very last lava flow occurred in 1761 AD. proposed by Lipman, 1997) and andesitic to dacitic lava flows (unit U2, in table 1); the top of the fill sequence The Cachiyacu area has been affected by intense glacial consists of laminated siltstone and sandstone in the lower erosion and surface deposits consists largely of till, which is facies and coarser grained sandstone and blocks of caldera covered by a periclinal 2m thick black soil containing filling lavas towards the upper sedimentary facies. discrete distal tephra layers of Holocene age.

Fig. 3 A-A’ geologic cross section

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Tabl e 1 . Cachiyacu Rock Chemistry Mayor element s reported in weight %, trace elements in ppm Ro c k Ty p e U1 U2 U3 U4 U5 Sam ple V-210 E05020 V-52 E05028 V-50 V-263 V-269 V-303 V-004 H-1 H-2 E05032 E05138 V-295 SiO2 57,59 75,01 59,25 63,06 68,80 65,70 68,42 68,46 56,67 63,83 59,20 62,17 61,70 62,98 Al2O3 16,48 12,36 16,60 16,18 15,20 15,88 15,48 14,05 16,24 15,31 15,20 16,60 16,29 15,70 Fe2O3 7,52 0,81 6,96 5,51 3,38 4,32 3,15 1,80 7,39 5,32 5,25 5,52 5,74 5,40 MgO 4,62 0,08 3,46 2,53 0,79 1,79 0,82 0,56 4,84 1,17 4,53 2,63 2,93 2,63 CaO 6,76 0,44 5,95 4,94 2,40 3,96 2,23 1,39 7,14 3,71 5,17 5,17 5,44 4,60 Na2O 3,76 4,24 3,93 3,78 4,39 4,19 4,40 3,50 4,01 4,10 2,93 4,32 4,25 3,96 K2O 1,16 4,26 1,97 3,55 3,28 2,89 3,39 3,88 1,88 2,70 2,17 2,40 2,36 3,26 TiO2 0,73 0,14 0,85 0,69 0,42 0,55 0,49 0,31 0,90 1,00 0,93 0,76 0,78 0,72 P2O5 0,15 0,69 0,22 0,39 0,13 0,16 0,18 0,08 0,39 0,29 0,28 0,40 0,41 0,25 MnO 0,10 0,05 0,09 0,07 0,05 0,08 0,06 0,06 0,10 0,08 0,10 0,08 0,09 0,08 LOI 0,80 0,73 0,40 0,67 1,00 0,20 1,20 5,70 0,00 -0,16 -0,21 0,10 Total 99,67 98,81 99,68 101,37 99,84 99,72 99,82 99,79 99,56 97,51 95,76 99,89 99,78 99,68 Rb 25,80 141,10 57,30 82,70 106,30 86,00 108,40 141,20 53,90 79,00 61,00 61,90 64,80 131,30 Sr 529,40 37,00 645,10 541,00 336,70 505,00 365,00 274,70 1060,50 419,00 645,00 783,00 779,00 719,80 Y 11,10 17,50 14,20 14,90 13,70 10,50 14,10 11,80 13,70 23,00 19,00 10,80 11,50 14,70 Zr 91,50 102,00 143,80 164,00 156,80 123,50 169,00 176,00 123,00 225,00 188,00 173,00 148,00 193,50 Nb 4,50 13,00 8,10 8,90 10,10 8,40 11,30 11,60 9,30 10,90 10,00 9,00 8,90 11,60 UTM(E)(08) 8,12 1,12 1,21 1,07 0,75 0,90 1,21 8,08 8,12 1,56 UTM(N)(99) 6,25 5,98 5,86 6,51 5,09 5,15 4,92 6,54 6,25 5,06 U1: pre caldera lavas and ignimbrites. U2: Initial (syn-) and post- collapse caldera fill (0,8 Ma). U3: Lava flows, domes, ignimbrit es post collapse (~0,45 - 0,16 Ma). U4: Lava flows (40 -20 Ka). U5: Historical lava flows. V-#: Fabián Villares for Electroguayas thesis in prep.; E05#: Chiaradia et, al 2009; H-#: Hall 2009, personal comunicat ion.

Active faults comprise the regional NNE dextral strike-slip on the geologic map (fig. 2) and its composition and system which marks the E limit of the NAB and likely physico-chemical parameters are given in Table 2. The controls the geometry of the Chacana caldera structure; it is springs are the only surface thermal manifestation in the also a most likely control on permeability for the deep whole caldera; other active features such as geothermal fluids. Faults and dykes of nearly EW strike are common; fumaroles and steaming ground are absent. Most of the several volcanic vents seem to follow a NS direction. The springs are distributed along a NE-striking fault (see fig. 2) Antisana NW lineament is a dominant feature S of on a total distance of 22 km; other springs are located on Cachiyacu, although is not reported as active. the western slope (E-9) and distant foothills (E-10;-11;-12) of the Chacana edifice.

The group of hottest and most abundant spring waters occur in the Cachiyacu area at 3900 m elevation on the caldera floor and associated with the Yanaurcu and San Clemente felsic domes. The total number of springs in unknown, but at least a dozen were mapped and three of them were sampled and analyzed (E-13;-14;-16). The spring water is crystalline, with a slight H2S odor, and deposits travertine in aprons and modest size terraces above river level. Temperature ranges from 40 to 64.6 °C and pH varies between 5.99 and 6.53; conductivities range between 3300 to 5700 micro siemens per cm (Fig. 5) indicating high salinities of up to 5000 mg/litre. The waters belong to the family of Cl-SO4 alkaline waters (Fig. 6).

A second group of springs (E-4 and E-7) is located in the Fig. 4 SiO2 vs K2O diagram for selected rock samples Tambo river valley, about 5 to 6 km NE of Cachiyacu, at of Cachiyacu 3500 m elevation, and associated with glaciated, low profile rhyolitic domes. Their temperature is slightly less than the 4. GEOCHEMISTRY OF CACHIYACU PROSPECT previous group (57 to 60 °C), although conductivity is up to Between one and two dozen of warm and hot springs crop 7250 micro S /cm. They belong to the same family of Cl- out in the Cachiyacu Prospect, most of them inside the SO4-alkaline waters; these springs are used for swimming caldera topographic margin. Location of springs is depicted pools.

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Table 2. Cachiyacu Spring Chemistry (ppm) Lateral outflow to NW Meteoric Papallacta - Jamanco springs Cachiyacu springs and SW recharge Sample E1 E2 E3 E4 E7 E13 E14 E16 E9 E19 E11 E5 E18 Elev. (masl) 3334 3304 3278 3518 3550 3918 3926 3910 3426 3458 2571 3455 4045 UTM (E) 17,2 17,1 17,3 13,5 13,4 8,9 8,7 8,8 98,7 98,9 91,4 17,1 9,9 UTM (N) 60,0 59,9 60,0 58,5 58,4 54,8 54,6 54,8 61,8 49,0 68,1 60,9 43,3 Temp.ºC 58,9 47,2 54,2 60,8 57,8 60,1 40,5 64,6 27,2 29,9 37,7 9,0 7,5 ph 6,82 6,56 7,08 6,23 6,15 6,53 5,99 6,39 7,17 6,10 6,48 7,95 6,36 Condt. µs/cm 3980 2900 2170 6820 7250 5000 3340 5560 5720 1780 1540 105 284 Li 2,40 1,40 1,40 6,60 8,00 7,20 4,90 8,10 1,90 0,20 - - 0,07 Na 54,02 306,60 282,40 1136,80 1306,10 825,20 553,30 916,00 1123,91 238,00 209,90 2,50 10,40 K 10,2 5,9 6,3 42,6 59,8 73,5 50,8 79,8 53,6 26,6 26,2 0,8 3,5 Ca 312,4 201,6 116,5 294,1 278,1 99,6 119,8 106,4 244,5 60,1 58,1 16,6 10,0 Mg 3,6 2,1 1,7 8,1 11,2 25,4 19,4 33,1 52,9 68,7 81,3 2,43 6,1 Si 30 24 28 50 54 75 54 80 42 68 72 9 - B 6,90 4,10 3,70 18,16 19,80 17,40 11,20 19,00 19,80 5,10 3,30 0,01 - Cl 973,5 492,1 401,3 1871,8 2114,2 1247,1 801,2 1302,4 493,1 234,3 107,8 1,5 2,1 F 1,90 1,50 1,90 1,80 3,00 1,00 1,00 1,00 3,80 0,80 0,60 0,01 1,10 SO4 530,4 380,2 372 300,9 236,6 117,1 86,9 115,7 1307,5 2208 73,9 2,9 10,1 HCO3 79,3 109,8 100,7 454,5 576,5 518,5 549 672,2 1576,9 762,5 902,8 62,83 73,2 As 1,03 0,66 0,96 3,2 4,22 5,71 2,7 5,4 9,15 0,2 0,41 0,001 - TDS 2005,65 1529,96 1316,86 4188,56 4671,52 3012,71 2254,20 3339,10 4929,06 3672,52 1536,31 98,58 116,57

A third group of springs is located further 4 km to NE from the previous ones, in the Papallacta river valley at 3300 m elevation and at the structural caldera margin. Associated lithologies comprise dacite domes, flow –textured diatreme breccia and caldera wall rocks of andesitic and dacitic composition. Their temperatures vary from 47 to 59 °C and conductivities range from 2000 to 4000 micro S/cm, which is lesser than the other two groups and can be due to dilution. The waters belong to the same family of Cl-SO4- alkaline waters as the previous groups and are used intensely for bathing purposes, and at an incipient level for space heating.

12000 Cond 10000 E7 8000 E16 E4 6000 E13 E1 4000 E17 pH 2000 E2 E3 E5 E6 0 6,00 6,50 7,00 7,50 8,00 Fig. 6 Plot of Cachiyacu waters in the Langelier- Ludwig diagram. Fig. 5 Conductivity in micro Siemens/cm vs pH for Spring E-19, located 12 km SW of Cachiyacu and outside Cachiyacu waters. the caldera structure at 3400 m elevation, likely represents a lateral outflow from Cachiyacu along the NE-SW fault, Spring E-9, also called Tolontag or La Calera spring, is which is the same structure as for the Tambo and Papallacta located outside the caldera structure, on the outer slopes of NE outflows. Outcrop temperature of water reaches 29.9 °C the caldera edifice, at an elevation of 3400 m; it is 12 km and waters show medium salinity (1780 micro S/cm distant from Cachiyacu area in a NW direction and conductivity); host lithology are pre-caldera rhyolitic represents a likely outflow from Cachiyacu along a fault. ignimbrites. These waters belong to the family of Although measured temperatures are as low as 27 °C, these bicarbonate-alkaline waters, according to Langelier – waters show high conductivity and elevated concentration Ludwig diagram. of solutes (evaporation ?), and belong to the same family of Cl-SO4-alkaline waters as the Cachiyacu group. An A group of low- to medium-temperature springs (E-11 in associated white travertine deposit hosted in andesite lava table 2; temp. 37.7 °C) is located at 22 km NW of has been recently mined out and the conduits for water Cachiyacu, in the IAV at 2500 m elevation. They belong to ascent seem to be almost sealed up. 6 Beate et al. the bicarbonate-alkaline water family and it is likely that geothermometer estimates, in particular on the CH4-CO2 vs they represent a distal outflow (INE, 1985), along the same CO-CO2 diagram, indicate a good correspondence with fault as E-9, from the Cachiyacu system. estimated liquid geothermometers deep temperature for the two important spring-water groups, namely 250 °C for A last group of water samples ( E-5; -6; -18) are local Papallacta – El Tambo and 300 °C for Cachiyacu. surface waters at temperatures ranging from 7.5 to 10.5 °C, which represent the meteoric recharge to the deeper seated 5. DISCUSSION geothermal system. They belong to the family of bicarbonate earth-alkaline waters and their total dissolved Cachiyacu is a novel geothermal area in Ecuador, which salinity do not exceed 100 mg/litre. haven’t been considered in earlier geothermal assessments, mainly due to lack of data. With the newly acquired Na/1000 geological and geochemical data, we propose a preliminary Giggenbach Maturity Diagram geothermal model for Cachiyacu, which has to be tested in follow-up studies, mainly geophysics.

Mapping of the main volcanic features has led to a better understanding of the evolution of the Chacana Caldera, Full equilibrium which hosts the Cachiyacu prospect. Persistent volcanic 160° 140° 180° 120° 200° 100° activity has been active in the area for at least 2 – 3 Ma. It 220° 80° 240° constructed the voluminous Chacana edifice, leading to it’s 260° collapse and formation of the caldera depression, refilling 280° Partial equlibrium 300° this depression with pyroclastic deposits, landslide breccias E4-E7 E1-E2 and lava flows, causing resurgence, diatreme formation and E3 Immature waters emplacement of intracaldera dacitic and rhyolitic domes E13-E14 E9-E16 and dome-flows, accompanied by explosive activity of Dissolution average crust ignimbrite and plinian eruptions of regional extend, the Mg K/100 latter dated in about 160 ka; the last volcanic activity 0 255075100 occurred as recent as 240 years ago in the vicinity of Cachiyacu, following many long-reaching lava flows Fig. 7 Cachiyacu water samples on the Na-K-Mg extruded in the last pleni-glacial period (40 – 20 ka). All Giggenbach diagram these volcanic activity is evidence for the shallow-level The isotopic composition of dD and d18O in our samples emplacement of evolved, voluminous magma bodies below (Fig. 8) highlights a meteoric origin, although only a few Chacana Caldera, which constitute the primary heat source. samples indicate a moderate oxygen shift, suggesting Cachiyacu in particular is surrounded by dacitic and equilibration processes with host rocks. Samples plotted on rhyolitic domes and lava flows, which vary in age from the triangular K-Na-Mg diagram of Giggenbach (Fig. 7) about 0.45 - 0.16 Ma to recent, and represent interest as a indicate three different groups: a) immature waters located heat source (Bloomquist, 1995); such large rhyolite close to the Mg corner; b) waters at partial equilibrium (E1, intrusions are of primary interest as deeper targets for EGS E2, E3, E4 and E7, which correspond to the Papallacta and systems (Duffield and Sass, 2003). rio Tambo springs); and, c) waters plotting at the border Reservoir rocks in Cachiyacu are assumed to consists of between immature- and partial equilibrium waters (E13, burried pre-caldera lava flows and breccias of the Mio- E14, E16, E9, which correspond to the Cachiyacu area Pliocene Pisayambo Formation, as well as andesitic lavas of springs and the Tolontag spring). the early volcano cluster and wall rock facies, either collapsed or close to the structural caldera rim. The caldera -70 collapse structure as well as regional NE-striking faults δD should assure reasonable permeability to the deep -75 E5-E6 circulating fluids. The likely location of the up-flow could E3 -80 be close to Plaza de Armas rhyolitic dome-flow complex, E10 E7 E4 some what 2 – 3 km S of Cachiyacu hotsprings. Extensive E11 E17 -85 steam – heated hydrothermal alteration is widespread on E14 E13 E16 Plaza de Armas N flank and indicates self-sealing of the -90 E12 E9 upper parts of the system. Temperature of the deep fluids

-95 seems to be well in excess of 200 °C according to liquid- E8 18 E19 δ O and gas-geothermometers and the system appears to be -100 water-dominated; its location is likely to be rather deep than -14,00 -13,00 -12,00 -11,00 -10,00 shallow. Fig. 8 Isotopic composition of Cachiyacu waters The seal cap to the system is the before mentioned self- Liquid-geothermometers, in particular Na-Li and Na-K sealing at the southern part as well as the tuff and diatreme (Ellis & Mahon, 1977), were used to estimate deep breccia dominated caldera infill towards N. The diatreme temperatures only on b) and c). The Papallacta – Rio breccia is highly propylitized and clay rich, and its funnel- Tambo group rendered estimated temperatures of 210 – 230 shape geometry towards the central part of the caldera, has °C for the Na-Li geothermometer and 130 – 170 °C for the rendered this central part as impermeable. That’s the reason Na-K geothermometer. The Cachiyacu group of waters that there are not hotsprings in the whole central part, gave estimated deep temperatures of 270 and 210 °C, except along the NE-striking fault that connects Cachiyacu respectively. with Tambo and Papallacta, which crosses the diatreme breccia and several rhyolite domes (see fig. 3). Bubbling gas analysis from the same thermal waters indicate dominance of CO2. Preliminar gas- The recharge to the system consists of meteoric water, which is plenty available in the area. Recharge zones are 7 Beate et al. crests along the topographic rim of the caldera as well as – Quito for logistical support to Fabián Villares. Least but volcanic vents and domes at the rim or on the caldera floor. not last, we express our warmest thanks to the indigenous communities of El Tambo, Jamanco and Tolontag for The shortcoming at this stage of knowledge is the lack of kindly granting the permits to work on their land. absolute age determinations of rocks and waters and should be accomplished in future work, with the aim to put up a REFERENCES more completed geological-geochemical model to be checked with geophysics, i.e. magneto-telluric surveys. Almeida, E.: Alternativas para el desarrollo Since Cachiyacu is located in an ecological sensitive area, geotermoeléctrico en la República del Ecuador. Unp. the Antisana Ecological Reserve, it is mandatory to comply Tech. Report for INECEL. (1990). with all the regulations imposed by law, if the permit is Almeida E. et al: Modelo geotérmico preliminar de áreas granted to develop Cachiyacu towards tapping geothermal volcánicas del Ecuador, a partir de estudios químicos e energy for power generation. isotópicos de manifestaciones termales. Unp. Tech. Report, INECEL (1992). 6. CONCLUSIONS AND RECOMMENDATIONS Aspden, J.A and Litherland, M.: The geology and Mesozoic - Cachiyacu is a promising geothermal prospect collisional history of the Cordillera Real, Ecuador. hosted in the southern edge of the Quaternary Tectonophysics, v.205, (1992) 187-204pp. silicic Chacana caldera, located at the crest of Eastern Cordillera, about 65 km ESE of the Baldock, M.W.: Geology of Ecuador, explanatory bulletin Capital city of Quito. of the national geological map of the Republic of Ecuador 1 : 1000000 scale, DGGM-IGS, London - Persistent and voluminous andesitic and dacitic to (1982) 54pp. rhyolitic volcanic activity is the indirect evidence of the presence of big-size magma bodies, Barberi, F. et al.: Plio-Quaternary Volcanism in Ecuador. emplaced at shallow crustal levels below Chacana Geology Magazine 125 (1988), 1-14 pp caldera in general and Cachiyacu area in Beate, B.: La Geotermia: Conceptos Generales , particular. Aplicaciones y Estado Actual en el Ecuador. Est. Geogr. Vol 4. Corp. Edit.Nacional. Quito (1991). - A geothermal reservoir is assumed to exist in the Cachiyacu area, hosted in lavas and breccia of the Beate, B. et al.: Mio-Pliocene adakite generation related to 1 - 2 km thick pre-caldera volcanic sequence. flat subduction in southern Ecuador: the Quimsacocha Permeability is inherent to fractured lavas and volcanic center. EPSL 192 (2001) 561-570 pp. enhaced by active regional faults, with cross the Beate, B.: El flujo piroclástico de Chalupas como causante caldera structure along a NE-strike. de un desastre natural en el Cuaternario de los Andes - Estimated liquid and gas geothermometer Septentrionales del Ecuador. Primer Simposio temperatures of the deep fluids in Cachiyacu are Latinoamericano sobre Desastres Naturales. Quito. estimated in the range of 210 to 300 °C, which is (1985). well in excess of 200 °C. Beate, B. & Salgado, R. : Geothermal Country update for Ecuador, 2000 – 2005. 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Bourdon, E., et al.: Magmatic response to early aseismic - Cachiyacu’s location in an ecological-sensitive ridge subduction: Ecuadorian margin case and protected area needs the political decision of (Southamerica). EPSL 6457 (2002) 16 pp. the government to proceed with development, Chiaradia, M., Muntener, O., Beate, B. & Fontigne, D. : with the challenge to be environmentally benign Adakite-like volcanism of Ecuador: lower crust and economically feasible. magmatic evolution and recycling. Contrib. Mineral Petrol. Accepted Feb. 2009 AKNOWLEDGMENTS We thank Electroguayas for funding the study and making Clapperton C.M. And Vera R. : The Quaternary glacial possible the use of data at early stages. We are also sequence in Ecuador: a reinterpretation of the work of indebted to Geology Dept. of Escuela Politécnica Nacional Walther Sauer. Journal of Quaternary Research 1 (1986): 45 – 56. Eguez A., and Aspden J.: The Meso-Cenozoic Evolution of the Ecuadorian Andes. 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